1. Field of the Invention
The present invention relates to a method for manufacturing a planar display device using the field electron emitting phenomenon.
2. Description of the Related Art
FIG. 1 is a perspective view showing a part of the cross section of a planar display device using the field electron emitting phenomenon having a typical configuration.
The planar display device 15 is constituted so that a first and second substrates 1 and 2 each comprising a glass substrate are facing each other through a reinforcement spacer 3 by keeping a certain interval from each other. Peripheries of these faced substrates 1 and 2 are airtightly sealed by, for example, a glass frit through an insulating outer-peripheral frame 14 made of ceramic or the like. An airtight flat space is formed between the both substrates 1 and 2, an electron emitting portion 4 is formed at the side of first substrate 1, and a fluorescent screen 5 is formed at the side of second substrate 2.
On the first substrate 1, for example, a plurality of first striped electrodes (so-called cathode electrodes) 11 and a plurality of second electrodes (so-called gate electrodes for taking out electrons) 12 are arranged in parallel in the direction to be intersected each other (e.g. to be intersected perpendicularly to each other) and intersections are electrically insulated from each other through an insulating layer 7.
Moreover, a field-emitting type cathode 4 is constituted correspondingly to each of the intersections between the first and second electrodes 11 and 12. These field-emitting type cathodes 4 respectively have a cold cathode configuration in which, an opening 8 passing through the insulating layer 7 and the upper second electrode 12 is formed at the intersection between the first and second electrodes 11 and 12 as shown in FIGS. 2A and 2B, and an electron emitting portion (so-called emitter) 9 is formed on the lower first electrode 11 in the opening 8. In this case, a plurality of electron emitting portions 9 are arranged for each pixel (for each sub-pixel because phosphors R, G, and B serving as three sub-pixels constitute one pixel in the case of a color fluorescent screen).
A metal back layer 6 made of a thin film conductive layer is formed on the fluorescent screen 5 at the second substrate 2 side and a high acceleration voltage is applied to the metal back layer 6.
Moreover, because a required voltage is applied between selected electrodes of the first and second electrodes 11 and 12, electrons are taken out from the electron emitting portions 9 of the field-emitting type cathodes 4 arranged at the intersections and accelerated by the above acceleration voltage, to pass through the metal back layer 6, and to impact the fluorescent screen 5, and thereby, the screen 5 is made to fluoresce, and a fluorescent display such as an image display is realized.
The above field-emitting type cathode is formed by the film forming process including spin coating, printing, vacuum evaporation, sputtering, and CVD (chemical vapor deposition) and the so-called photolithography process including etching using a photoresist mask and lift-off.
FIGS. 3 and 4 show the steps of manufacturing a field-emitting type cathode according to a prior art.
First, as shown in FIG. 3A, a striped first electrode 11 is formed on one plane of a first substrate 1, an electron emitting portion 9 is formed at the intersection with a second electrode 12 on the first electrode 11 through the lift-off method or selective etching, thereafter an insulating layer 7 is formed on the entire surface, and moreover a striped second electrode 12 intersecting with the first electrode 11 is formed on the insulating layer 7.
Then, a positive-type photoresist layer 17 is formed on the entire surface including the second electrode 12 and only the photoresist layer 17 at a portion corresponding to an electron emitting portion 9 is selectively exposed by applying ultraviolet radiation 19 through a photomask 18. Reference numeral 17a denotes a portion to be exposed and 17b denotes a portion to be unexposed. In this step, the position of the photomask 18 is adjusted on the basis of a previously-formed reference marker so that the center of the electron emitting portion 9 coincides with the center of the opening of the second electrode 12 to be thereafter formed.
Then, as shown in FIG. 3B, development is performed to remove the exposed portion 17a of the photoresist layer 17 and form the photoresist layer 17 on which an opening 20 is formed.
Then, as shown in FIG. 4A, the opening 8 is formed with selective etching by using the photoresist layer 17 as a mask so that the electron emitting portion 9 is exposed with the openings passing through the second electrode 12 and the insulating layer 7 below the second electrode 12.
Then, as shown in FIG. 4B, the photoresist layer 17 is removed to obtain the field-emitting type cathode 4 constituted by forming the electron emitting portion 9 in the opening 8 formed at the intersection between the first electrode 11 and the second electrode 12.
In the case of the above conventional method for manufacturing the field-emitting type cathode 4, a substrate 1 is deformed due to a film stress generated when the insulating layer 7 is formed through sputtering and CVD in the steps of FIG. 3A or a relative positional shift is produced between the opening-forming photomask 18 and the position of the electron emitting portion 9 due to expansion and contraction of the substrate 1 caused by heat treatment of glass paste relating to printing when forming the insulating layer 7. Therefore, as shown in FIG. 4B, when a positional shift is finally produced between the opening 8 of the second electrode 12 and the electron emitting portion 9, problems occur that the number of electrons to be emitted fluctuates and irregular display appears.
On the other hand, when decreasing the distance between the electron emitting portion 9 and the second electrode 12, an electron emitting voltage tends to become lower. When the electron emitting voltage lowers, a display circuit becomes inexpensive and a display device at a low power consumption is realized. Therefore, very fine patterning is requested.
However, most exposure systems for manufacturing a large planar display device of 20 inches type or more use the so-called proximity exposure in which the photomask 18 is exposed by separating it from the photoresist layer 17 by considering the damage of the photomask 18. Because the proximity exposure is of a method to form a gap between the photomask 18 and the photoresist layer 17, it is a problem that a deformed substrate cannot be corrected and thereby, a positional shift occurs.
Moreover, because a gap is present between the photomask 18 and the photoresist layer 17, a disadvantage occurs that a very fine pattern cannot be obtained.
In the field of manufacturing of a semiconductor device such as an LSI, a projection system is used as an exposure device for realizing very fine photolithography. However, the projection system is not realistic because an exposure system is very expensive and an exposure system for a planar display device of 20 inches type or larger is restricted in its optical system.
By applying the self-alignment method to the alignment between the opening 8 of the second electrode 12 and the electron emitting portion 9, the problem of positional shift due to deformation or expansion and contraction of a substrate, which generates when forming the insulating layer 7, is solved. Moreover, because the number of photomasks and the number of position adjusting steps for exposure are decreased by the self-alignment method, an inexpensive planar display device can be manufactured.
As an example of manufacturing a field-emitting type cathode using the self-alignment method, the spin vacuum-evaporation method (so-called SUPINTO method) developed by Mr. SUPINTO in US at the SRI (Stanford Research Institute) is known.